1. Field of the Invention
The invention relates to the recovery of volatile organic compounds. More particularly, it relates to the improved recovery of such volatile organic compounds from vent gas streams.
2. Description of the Prior Art
It is well known that many industrial operations utilize volatile organic compounds (VOCs), including a variety of manufacturing, chemical production and petroleum refining operations. Generally, existing industrial processes and systems are equipped to capture a fraction of these VOCs, either for reuse within the process or for destruction. Some VOCs inevitably leave the process, however, and pass into the environment as a result of air emissions, waste water discharges, solid residue discards, and the like. The VOCs are usually present in relatively low concentrations in such waste streams removed from the industrial process.
As a result of heightened environmental awareness on the part of industry and the general public, recent legislation in many countries due to such environmental awareness, as well as economic penalties associated with the loss of valuable feedstock or product, there is an increased need to maximize the recovery of VOCs from dilute process streams. For example, the Clean Air Act Amendments of 1990 in the United States of America require a 90% reduction in emissions of 189 toxic chemicals by the year 2000. About 70% of these toxic chemicals can be properly classified as VOCs. The cost of implementing the required Maximum Achievable Control Technology (MACT) requirements is estimated to be in the billions of dollars.
Several technologies are commercially available for controlling volatile organic compounds. For example, vent gas condensation using cryogenic fluid or mechanically chilled refrigerant provides both the benefits of minimum emissions and solvent recovery. Adsorption processes using activated carbon or molecular sieves can be employed, but involve relatively complicated regeneration procedures. Other techniques, such as thermal and catalytic incineration techniques can destroy the volatile completely, but do not provide recovery of organic material. As a result, vent gas condensation is the most desirable waste control technique, since the VOCs can be recovered and reused.
However, most solvent recovery units that use mechanical chillers, generally operating at between 0.degree. and -40.degree. C., can no longer meet stringent emission standards. To improve VOC recovery, conventional batch cryogenic condensers can be added onto existing mechanical chillers. Such conventional batch cryogenic condensers require a long defrosting cycle since the VOCs are frozen indiscriminately to a preset freezing temperature. Dual condensers are required to ensure uninterrupted operation. To recover the volatiles, such condensers freeze all of the volatiles, including water. With organic material and water ice forming in the condenser wall, the unit is then defrosted periodically. The defrosting cycle is relatively long, and cryogenic fluid consumption is very high.
Mechanical chillers, as indicated above, can be used to condense VOCs and are less expense to operate than cryogenic vent gas condensers, but the equipment required is more expensive. Furthermore, as also noted above, mechanically chilled condensers have restrictive temperature limits. Ammonia chillers can only reach -40.degree. C. Freon chillers will likely be banned from commercial operations for environmental reasons. To meet the stiff emission requirements increasingly applicable for industrial applications, VOC recovery units capable of reaching lower operating temperatures are required. Mechanical chillers combined with presently available vent gas condensers provide a stopgap measure for satisfying emission control requirements. However, this approach is not economically attractive, since two separate pieces of equipment are needed to accomplish the required emission control task.
It should be noted that the operating efficiencies of batch condensers generally are rather poor. All batch condensers tend to have one preset temperature. At this preset temperature, a majority of the low boiling components of a vent stream will be condenses, but all of the high boiling components will be frozen. Frequent freezing and defrosting cycles consume excessive amounts of energy and refrigerant, with the refrigerant being spent to cool the defrosted condenser back to cryogenic temperatures. Reductions in the freezing-defrosting cycles would improve the economics of the VOC recovery process.
During the cooling cycle, batch condensers are also slow to recover waste refrigerant, due to the large imbalance in heat load. In a condensing vapor stream, more energy is spent on overcoming the heat of condensation and the heat of solidification, rather than in removing existing sensible heat. As the condensed liquid or frozen solid remains on the condenser surface, the exhausting clean vent gas has too little enthalpy to heat exchange with the incoming vent gas. Consequently, the incoming vent gas can usually be pre-cooled only to a temperature far above the boiling point of the volatiles, e.g. methanol with a boiling point of 65.degree. C. With sufficient thermal gradient, spent nitrogen would be vented at about 20.degree. C. below the freezing point of the volatiles, e.g. methanol with a -98.degree. C. melting point, so nitrogen would vent at -118.degree. C.
As an alternative, an economizer can be used to recover waste refrigerant from a spent nitrogen stream. However, condensation or freezing generally are not allowed in economizers. The incoming vent gas can be pre-cooled only to a temperature above the boiling point of the volatile material. With excess heat load respecting the spent nitrogen, it is vented cold, e.g. at -20.degree. C. No heat recovery can be performed with the exhausting clean vent gas, which is vented at the freezing point of the volatiles, e.g. at -118.degree. C.
Batch cryogenic condensers also have significant disadvantages when handling gas streams containing multiple component volatiles. For example, water vapor can condense rapidly on the condenser coil. Because of the large heat of condensation and heat capacity available, a large volume of water vapor can temporarily stop the organic vapor from condensing, resulting in an undesired release of organic vapor to the atmosphere, As the cryogenic vapor catches-up with the temperature rise, the organic vapor starts to condense, but water starts to freeze. Under such circumstances, it is not long before the condenser systems loses its heat transfer capability, due to the presence of large blocks of ice in the coils of the condenser.
Under such existing circumstances, it will be appreciated that further developments are desired in the art to enable applicable clean air standards to be complied with in an efficient and economical manner. The development of a continuous vent gas condenser approach, not requiring dual operating units, would represent a significant advance in the art of vent gas control and recovery.
It is an object of the invention, therefore, to provide an improved method for recovering volatile organic compounds from vent gas streams.
It is another object of the invention to provide a continuous method for removing and recovering multiple organic volatiles from a vent gas stream.
It is a further object of the invention to provide a method for recovering organic components from vent gas streams so as to obviate the need for employing cryogenic condensers in combination with mechanical chillers for VOC recovery from vent gas steams.
With these and other object in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.